Objective Phyllosphere microbial communities have important impacts on forest ecosystem stability and plant health. This study aims to reveal the effects of light intensity and plant life forms on phyllosphere microbial communities of understory vegetation, providing theoretical support for urban forest ecological management and ecosystem stability.

Method Typical shrubs and herbs under Pinus tabuliformis plantations in Beijing Xishan National Forest Park were selected as research subjects. Based on photosynthetically active radiation (PAR), three light intensity gradients were established: low (< 500 μmol/(m²·s)), medium (500 ~ 1 000 μmol/(m²·s)), and high (> 1 000 μmol/(m²·s)). Using high-throughput sequencing, diversity, community structure, and amplicon sequence variant (ASV)-specific response characteristics of phyllosphere bacterial and fungal communities were analyzed.

Result (1) Light intensity had no significant effects on α-diversity of bacterial and fungal communities, whereas plant life forms significantly affected microbial diversity and structure, especially fungi. Chao1, Observed_species, and Good’s_coverage indices of fungal communities in herbs were significantly higher than those in shrubs (P < 0.05). PERMANOVA results showed plant life forms explained 3.83% and 3.70% of variations in bacterial and fungal communities, respectively (P < 0.001). (2) Community composition analysis indicated bacterial communities were dominated by Proteobacteria, and their dominant genera remained stable across conditions. However, fungal communities varied significantly at the genus level with light intensity and plant type, with Phoma dominating under low-light conditions and Alternaria under medium and high light. PCoA and PERMANOVA analyses further verified the significant impact of plant life forms on microbial community structure (P < 0.001), while the effect of light intensity was not significant. (3) ASV analysis indicated high specificity among life forms and plant species; fungal communities shared 18.6% of ASVs between shrubs and herbs, while bacterial communities shared only 11.47%, indicating even higher specificity. At the species level, ASVs were mostly species-specific, showing strong host specificity.

Conclusion  Plant life form is a key driver of phyllosphere microbial community structure in understory vegetation, with a more pronounced effect than light intensity, especially on fungal assemblages. While bacterial communities remained relatively stable, fungal composition varied across light conditions and host types. These findings highlight the ecological role of life form in shaping microbial assemblages, offering a foundational reference for future research and the ecological design of urban forest systems.